D) To establish boundaries in the event of a nuclear war
The American government control part of America but they don't control all of it
They thought they were doing something for the benefit of the Southern society at the time.What could be wrong with killing what was supossedly damaging the white society.The murder was committed as an act of cleansing, that is why they considered themselves heroes.Some Southeners believed that there were evil intentions in the Black, they thought of them as impure and revengeful they did not belong to the world of the free white Americans.
The Copperheads were democrats from the north that wanted peace.
It was true that many troops were sent to Mexico based on the announcement that president Trump had made together with the Mexican president. According to their agreement terms, they signed an agreement that allowed the troops to go to Mexico and support their forces in ending the crisis that had engulfed for a long time.
The negotiation was mainly about the exports, and the troops were to protect the borders through the laws that had been passed to protect the data that will enter and leave a given country subsequently.